A separation device of this kind is known for instance through U.S. Pat. No. 1,927,822, U.S. Pat. No. 3,424,375 or EP 0 047 677 A2. The separation device can be applied directly onto a container containing the liquid to be cleaned.
Often it is not possible to keep the liquid surface in a container for liquid to be cleaned constantly at a predetermined level. Upon use of a separation device of the above said previously known kind the tubular inlet member, in a case like this, will be more or less immersed in the liquid. Since the inlet member has to extend under the liquid surface, when the liquid is at a relatively low level, this means that an undesired large part of the inlet member will be immersed in the liquid when the liquid surface is at a relatively high level.
One reason why the rotating inlet member should not be immersed deeper than necessary in the liquid to be cleaned is that this causes rotation of the liquid in the container. This reduces the pumping effect of the inlet member and causes undesired splitting of particles, which later are to be separated from the liquid in the centrifugal separator. Another reason is that an unreasonably high amount of energy is required to operate the centrifugal rotor.